Generally, beam forming is where transmission or reception of signal power is concentrated in the direction of an intended respective receiver or transmitter. Both the transmission and reception of signals can benefit from beam formed patterns compared to omni-directional patterns. From a transmitter's perspective, beam forming lessens the power needed to perform the transmission, and lessens the power causing interference directed to nonintentional receivers. From a receiver's perspective, beam forming enhances the desired received signal and lessens the interference due to other transmitters or signal sources the further they are removed from the primary axis of a transmission.
Referring to FIG. 1, beam forming is usually associated with fixed infrastructures such as microwave towers 10, 12, for example. This is because it is relatively easy to point static transmit and receive beams of microwave towers 10, 12 toward each other as shown in FIG. 1. The structures on which the towers are mounted and the beam widths that are utilized simply need to be sufficiently stable so that the beams remain overlapped thereby providing reliable transmission. If towers or beams are unstable due to structural or electrical instabilities, the beams may not adequately overlap as shown in FIG. 2. In such situations, however, correction is relatively easy because the beams are transmitted from two fixed locations and the degree of misalignment is typically relatively minor.
With rapidly increasing capacity and coverage requirements of wireless communication systems, however, beam forming may also be used between base stations and wireless transmit/receive units (WTRUs). Referring now to FIG. 3, there is shown a base station 20 that is using beam forming and a WTRU 22 that is using an omni-directional pattern. Ignoring possible external influences on the beam (i.e. physical obstructions), the base station 20 should have a reasonably static pattern position. The WTRU 22, on the other hand, is subject to rotation and location movement in any direction. If the transmission pattern of the WTRU 22 is truly omni-directional (i.e. approximated by a circle), rotation will have no effect on the communications link. Location movement, however, will pose a problem in that it can change the relationship of the WTRU 22 and base station 20 communication link. For example, in FIG. 3, WTRU 22 is initially emitting omni-directional pattern 24 and then changes location and begins emitting omni-directional pattern 26. The base station 20 may therefore need to modify its beam to maintain contact. Extreme changes could of course require switching to another base station, which is called handoff (or handover) and occurs naturally in existing wireless communication systems.
Referring to FIG. 4, the base station 30 is using an omni-directional pattern and the WTRU 32 is using beam forming. Here, a further problem is introduced in that, because the WTRU 32 is using beam forming, location movement as well as rotation can now deteriorate the pattern overlaps between the base station 30 and WTRU 32. For example, in this situation, WTRU 32 is initially emitting beam pattern 34 and then changes position as a result of rotation or location movement or both and begins emitting beam pattern 36. This situation, however, can also be handled using handoff which, as mentioned, is an existing capability of existing systems. It should be noted that the omni-directional pattern of the base station 30 could be replaced by a sectored pattern as is often found in wireless systems. The key point is that the base station 30 is providing complete coverage surrounding its location so that while rotation and location movement of a WTRU 32 may require handoff between sectors, this is an existing capability of existing wireless systems.
As shown in FIG. 5, however, where both entities (i.e. a base station and WTRU) are using beam forming, movement by a WTRU 40 (see dashed patterns) is more likely to disrupt the pattern overlap. That is, while beam forming improves communications when properly aligned patterns are used, misalignment is more likely where both WTRUs and base stations use beam forming thereby making link establishment and maintenance more time consuming and difficult.
For example, in FIG. 6, the “before adjustment” situation shows two misaligned beams. In the prior art, the entities from which those beams originate Xa, Xb (both of which may be a base station or WTRU) both determine an adjustment to better align the beams, but since they are not aware of what the other is doing, they both perform the required adjustment. The net adjustment, therefore, causes a resultant error in alignment that is roughly equivalent to the original error, but with the beams pointing in different directions as shown in the “post adjustment” situation. The next time the adjustments are attempted, the same thing can happen thereby causing the beams to fall into an oscillating pattern around the optimal alignment of the beams. It is important to note that there is no implied timing relationship between the measurements or actual adjustments of the beams. Therefore, the only situation required to cause this problem is that the measurements made by one entity and the resultant adjustment that is performed are time overlapping with the same measurements and resultant adjustment occurring at another entity.
What is needed, therefore, is a method and system for coordination of beam forming in wireless communication systems.